To read the verse as though it says once the fullness of the Gentiles has come in then the blindness to Israel shall be lifted, is reading our doctrine into the passage. Paul tells only that blindness in part had come to Israel. Why is the blindness only in part? Because there has always been a saved remnant from Israel that were never in blindness.
The question that challenged me is how can all Israel that shall be saved be speaking of ethnic Jews since their fullness comes from Gentiles being grafted into the same good olive tree of faith with them? Why do we assume Paul saying the blindness in part will then be lifted? That's not what he says. He says only that Israel is in partial blindness. It is Paul's desire for his kinsmen and us to understand "this mystery" that has been made known since the advent of Christ. The mystery is that Gentiles of faith and Jews of faith together is how "all Israel" shall be saved. Salvation through Christ is not only to Jews, but Gentiles too, and when the last Gentile is saved, the seventh trumpet will sound the 'mystery' of God finished, and that time should be no longer.
Romans 11:25-26 (KJV) For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel, until the fulness of the Gentiles be come in. And so all Israel shall be saved: as it is written, There shall come out of Sion the Deliverer, and shall turn away ungodliness from Jacob:
Revelation 10:5-7 (KJV) And the angel which I saw stand upon the sea and upon the earth lifted up his hand to heaven, And sware by him that liveth for ever and ever, who created heaven, and the things that therein are, and the earth, and the things that therein are, and the sea, and the things which are therein, that there should be time no longer: But in the days of the voice of the seventh angel, when he shall begin to sound, the mystery of God should be finished, as he hath declared to his servants the prophets.
When the last Gentile to be saved together with Jews of faith, the Kingdom of God will be complete as the Gospel is sent unto all the nations of the earth and whosoever believes the Gospel has everlasting life through Him. Then all "Israel of God" shall be saved. Not an ethnic people only, but all people of faith as one body in Christ. There will not be more time given this earth whereby man must be saved. The end of time has come and the only time remaining will not be for being saved but will be for Satan's "little season."
Galatians 6:15-16 (KJV) For in Christ Jesus neither circumcision availeth any thing, nor uncircumcision, but a new creature. And as many as walk according to this rule, peace be on them, and mercy, and upon the Israel of God.
Your doctrine leaves the timing up to lost Gentiles. You are literally saying when that last Gentile finally decides to repent, and receive the gift of Salvation, the 7th Trumpet will sound, because that last Gentile is the blindness removed.
That was not Paul's point at all.
Only God can remove the blindness, because He placed that blindness on Israel, and that blindness started when the 10 northern tribes were scattered across the earth. Blindness in part was not just being blinded to the Gospel, but to all of God's plan, which included being cut off from the Olive Tree as given in the analogy.
The blindness was in part, because even those scattered across the earth could come to God in repentance, even before the Messiah arrived. The "completed church" (that last Gentile, according to you) is not the end of time. The end of time is the 70 weeks as the mystery given in the prophets. The Gospel was not the mystery. The mystery was given to the people of Israel, so the mystery would be that God extended His elect outside of Israel, to include all of Adam's offspring.
That is the point of natural branches and wild branches. Israel considered themselves as only the elect. The mystery was that all of Adam's offspring was included in the elect, not just Israel. We can see this sentiment in these verses:
"And, behold, a woman of Canaan came out of the same coasts, and cried unto him, saying, Have mercy on me, O Lord, thou son of David; my daughter is grievously vexed with a devil. But he answered her not a word. And his disciples came and besought him, saying, Send her away; for she crieth after us. But he answered and said, I am not sent but unto the lost sheep of the house of Israel. Then came she and worshipped him, saying, Lord, help me. But he answered and said, It is not meet to take the children's bread, and to cast it to dogs. And she said, Truth, Lord: yet the dogs eat of the crumbs which fall from their masters' table. Then Jesus answered and said unto her, O woman, great is thy faith: be it unto thee even as thou wilt. And her daughter was made whole from that very hour."
This was a real event, but Jesus was not being rude because He was God. He was demonstrating how they misunderstood the mystery. Even the Gentiles understood how they were viewed by Israel.
In the anology of the OP, Paul was not saying Israel became wild branches, nor that the wild branches became natural branches, yet the doctrine of many want to blur those lines, because Paul also taught that in Christ there was no difference.
So the anology still went along with the mystery as there was a distinction between the natural branches and the wild branches. That mystery was deliberately placed on Israel, by God, at some point prior to even the Babylonian captivity, that Daniel experienced, and applied only to Israel, not necessarily to Gentiles.
Paul even said the Gospel made them (Israel) enemies of the church. But according to election way before even Abraham, no nations were enemies towards each other. That God called Israel out of the world and made them distinct natural branches should not have been a mystery, but a blessing for all who submitted to the Law including Gentile converts. It became a mystery to Israel when God placed them into the partial blindness. That would mean it would be hard for Israel to accept the Gospel, like it was hard for a Gentile to convert to the Law of Moses. Not impossible, and not that Gentiles could make Israel feel insignificant, like being a Gentile under the law made them inferior to Israel.
If there is no distinction, why would you even make a point "that the last Gentile...". That seems hypocritical. What if the last person saved is a descendant of Jacob? This has nothing to do with God waiting for the last person to make up their mind.
There will be many who could have, but did not, remember the 5 wise and 5 foolish. You make it sound like God was waiting for that last 5th wise person to have oil. Why? Jesus will come ready or not, then the partial blindness will be removed, and then many of Israel will be made righteous after the church has gone. But those will be the firstfruits of an earthly kingdom, not part of the church, as that opportunity was taken away at the Second Coming, ready or not.
Yes, the fulness of the Gentiles will be brought in. That is based on God's timing, not a single holdout Gentile, marking the end of being prepared.
